1. Field of The Invention
The present invention relates to an optically addressed spatial light modulator having a function which can input two-dimensional optical information such as an image and a data pattern by using write light, and can display them by readout light. In particular, the present invention relates to an optically addressed spatial light modulator which can be applied to a projection image spatial light modulator which converts a weak optical image to a bright image, a wavelength convertor of visible optical images, a visualizing device of infrared light images, a fire alarm device, and a convertor from incoherent to coherent light and vice versa.
In recent years, the following spatial light modulators (1)-(4), comprising a photoconductive layer and a liquid crystal layer, were used as optically addressed spatial light modulators.
(1) A spatial light modulator is shown in FIG. 6 wherein a twisted nematic liquid crystal layer 20, with a 45.degree. twist, is sandwiched by two alignments 19,19. Initially, a multilayer dielectric mirror 17 for reflecting light, a light absorbing layer 18, and an amorphous inorganic photoconductive film 1B, comprising amorphous Si are sequentially fixed onto one alignment 19, and then transparent substrates 11 having transparent electrodes 9 are fixed to the sides of the device. In FIG. 6, a number 10 indicates an AC power, and a number 12 indicates a lead wire.
(2) A spatial light modulator wherein alignments, and photoconductive polymer (i.e. polyimide) films are sequentially fixed to both sides of a 90.degree. twisted nematic liquid crystal layer. Then, transparent substrates 11 having transparent electrodes 9 are fixed to the sides of the device.
(3) A spatial light modulator wherein photoconductive polymer crystal films (i.e. polyimide integrated into oligophenylene sulfide which is sensitive to light) are sequentially fixed to both sides of a surface stabilized ferroelectric liquid crystal layer. Then, transparent substrates 11 having transparent electrodes 9 are fixed to the sides of the device.
(4) A spatial light modulator wherein a multilayer dielectric mirror, a light absorbing layer, and an inorganic photoconductive crystal plate (Bi.sub.12 SiO.sub.20) are sequentially fixed onto one surface of a layer which is made of a liquid crystal/resin composite (liquid crystal/resin composite layer), and then transparent substrates 11 having transparent electrodes 9 are fixed to the sides of the device.
The disadvantages of the spatial light modulators (1)-(4) are presented hereinbelow.
1. Because the spatial light modulator (1) uses a twisted nematic liquid crystal in the liquid crystal layer, then the thickness of the liquid crystal layer has to be maintained accurately, and therefore it is difficult to produce the spatial light modulator having large dimensions.
2. Because the spatial light modulator (1) uses a twisted nematic liquid crystal in the liquid crystal layer, then a polarizer is necessary. Because in the case of displaying an image by non-polarizing readout light, more than 50% of light is removed by the polarizer, and consequently the displayed image is dark.
3. Because the spatial light modulator (1) uses a twisted nematic liquid crystal in the liquid crystal layer, then the liquid crystal layer is easily deformed and the thickness thereof is malformed easily by stress from external pressure.
4. Because the spatial light modulator (1) uses an amorphous Si is used in the photoconductive layer, then a extensive manufacturing facility such as a chemical vapor deposition (CVD) apparatus, a sputtering apparatus, and a vacuum evaporation apparatus is necessary. Moreover, the length of time for producing a film is long, therefore the efficiency of producing a spatial light modulator is greatly reduced.
5. Because the spatial light modulator (1) uses an amorphous Si is used in the photoconductive layer, then the photoconductive layer has low sensitivity to an infrared ray. Therefore a light beam made by a cheap semiconductor laser cannot be used as a write light.
1. Because the spatial light modulator (2) uses a twisted nematic liquid crystal in the liquid crystal layer, then a production of a spatial light modulator having a large dimensions is difficult.
2. Because the spatial light modulator (2) uses a twisted nematic liquid crystal in the liquid crystal layer, then a polarizer is necessary and, consequently, the displayed image is dark.
3. Because the spatial light modulator (2) uses a twisted nematic liquid crystal in the liquid crystal layer, then the liquid crystal layer is easily deformed by stress from external pressure.
4. Because the spatial light modulator (2) uses a single layer of film made by an organic photoconductive material, the mobility of carriers is low, and therefore the practical sensitivity is inferior.
1. Because the spatial light modulator (3) uses the surface stabilized ferroelectric liquid crystal in the liquid crystal layer, the thickness of the liquid crystal layer has to be controlled extremely thin (i.e. 1 to 3 .mu.m), and therefore it is difficult to produce a spatial light modulator having large dimensions.
2. Because the spatial light modulator (3) uses the surface stabilized ferroelectric liquid crystal in the liquid crystal layer, a polarizer is necessary, and consequently the displayed image is dark.
3. Because the spatial light modulator (3) uses the surface stabilized ferroelectric liquid crystal in the liquid crystal layer, the alignment state of liquid crystal is extremely unstable against mechanical shock, and therefore a display defect easily accrues.
4. Because the spatial light modulator (3) uses the surface stabilized ferroelectric liquid crystal in the liquid crystal layer, it is difficult to form an alignment layer which can orient the liquid crystal molecules uniformly, therefore the spatial light modulator having a uniform property cannot be obtained.
5. Because the spatial light modulator (3) uses the surface stabilized ferroelectric liquid crystal in the liquid crystal layer, the liquid crystal layer has a bistable electric-optic property, and therefore a display having gray scale cannot be obtained.
6. Because the spatial light modulator (3) uses a single layer film made by an organic photoconductive material 1A, the mobility of the carriers is low, and therefore the practical sensitivity is inferior. 1. Because the spatial light modulator (4) uses a single crystal plate, comprised of Bi.sub.12 SiO.sub.20, in a photoconductive layer, a light comprised of a long wave length such as an infrared ray and a visible ray, cannot be used as a write light. Therefore the spatial light modulator cannot use the various light sources of the write light.
2. Because the spatial light modulator (4) uses a single crystal plate, comprised of Bi.sub.12 SiO.sub.20, in a photoconductive layer, a polishing process is needed for the photoconductive layer, and therefore the productivity of the layer is low.
3. Because the spatial light modulator (4) uses a single crystal plate, comprised of Bi.sub.12 SiO.sub.20, in a photoconductive layer, the photoconductive layer is fragile and easily broken when subjected to mechanical shock or oscillation.